I agree about slow players being the most irritating, but they have to be really slow to get on my nerves. I'm talking about 2-5 minutes per shot sometimes. I usually don't take more than 20 seconds on most shots when I'm in a high-pressure match. The shots that I do take longer on are the key shots of every rack, which is usually only one shot. On those I may take about 30-45 seconds, depending on how tough it is. Alot of times even though I know what to do, I just kind of "gather" myself on those key shots, because I know if I make it and get the position I need, I'm usually out from there.
One way to spot an inexperienced player is someone who plays faster and faster with every shot they make. It's kind of like they are trying to show off, sort of trying to "wow" the spectators or their opponent. They usually miss anyhow. Having a steady rythm is very important. Being able to keep that rythm when you are playing a lightning-fast player or super slow player is very tough to do, but it usually prevents your game from breaking down. A friend of mine taught me that, when playing very slow playrs, when sitting in your chair, focus on your breathing. Just take deep belly breaths and try to relax. When you get to the table, slow your pace down a little bit, and keep that pace the rest of the match. A good way to do that is to walk around the table, on every shot. Shoot, take a walk around the table looking at your options, decide what you're gonna do, and shoot again. This is a slow rythm, but it is a good rythm when your opponent's speed is driving you nuts. This also is a good rythm to have when playing against super fast players, it's kind of like turning the tides on them, because sometimes players shoot really fast to screw up your pace.